1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to golf balls having a core and a cover, with dimples being formed on the cover.
2. Description of the Related Art
General golf balls that are commercially available in the market have a core and a cover. There exist cores composed of a single solid rubber layer, those composed of two or more solid rubber layers, and those composed of a synthetic resin layer in addition to a solid rubber layer, and the like.
There are numerous dimples formed on the surface the cover. A role of the dimples involves causing turbulent flow separation through disrupting the air flow around the golf ball during the flight (hereinafter, referred to as “dimple effect”). By causing the turbulent flow separation, a separating point of air from the golf ball shifts backwards leading to the reduction of a drag coefficient (Cd). The turbulent flow separation promotes the differentia between upper and lower separating points of the golf ball that result from the backspin, thereby enhancing the lift force that acts upon the golf ball. Flight distance of the golf ball is prolonged on behalf of the reduced drag and enhanced lift force. Aerodynamically excellent dimples promote the turbulent flow separation. In other words, aerodynamically excellent dimples may render the air flow better.
What is most demanded for a golf ball by golf players is travel distance. Golf players place particular importance on the travel distance upon hit with a driver. Specifications of dimples that may exert a great influence on flight performance of a golf ball include total volume. Golf balls having too small total dimple volume tend to provide a hopping trajectory, whilst golf balls having too large total dimple volume tend to provide a dropping trajectory. In either case, sufficient flight distance is not achieved. In order to achieve a proper trajectory and a great flight distance, total volume of the dimples must be defined within a predetermined range.
Various proposals have been made in connection with density of dimples. JP-B No. 58-50744 discloses a golf ball having densely arranged dimples such that a pitch between the dimples is as 1.62 mm or less as possible. JP-A No. 62-192181 discloses a golf ball having densely arranged dimples such that any new dimple having greater area than the average area can not be formed. JP-A No. 4-347177 discloses a golf ball having extremely densely arranged dimples, with lands, on which a rectangle having a predetermined size can be drawn, existing by equal to or less than 40. Any one of the golf balls disclosed in these known documents has densely arranged dimples, in other words, it has increased surface area occupation ratio of dimples. Persons skilled in the art acknowledge that surface area occupation ratio is an important factor that exerts an influence on the dimple effect in addition to total volume of dimples.
Performances of golf balls on which great importance is attached similarly to flight distance include a spin performance. High backspin speed results in small run (a distance from the position where the golf ball dropped to a position where it stopped, also referred to as “roll”). In other words, for golf players, golf balls which are liable to be spun backwards are apt to be stopped readily at a targeted position. High side spin speed results in possibility of curving of the golf ball. In other words, for golf players, golf balls which are liable to be side spun are apt to be intentionally curved. Golf balls that are excellent in spin performances are excellent in control performances. High-class golf players particularly place great importance on control performances upon impact with a short iron.
In general golf balls, an ionomer resin has been used in the cover. Ionomer resins are excellent in durability and the resilience performances. However, ionomer resins have high hardness, in general. Golf balls having a cover composed of an ionomer resin are inferior in control performances. In an attempt to improve the control performances, a cover in which a hard ionomer resin and a ternary copolymer-based soft ionomer resin are used in combination has been proposed. Furthermore, a cover in which an ionomer resin and a thermoplastic elastomer are used in combination has been.also proposed. In addition, a cover composed of a thermosetting polyurethane has been also proposed.
JP-A No. 2002-360740 discloses a golf ball having a cover including a thermoplastic polyurethane elastomer as a principal component. This cover is soft. This golf ball is extremely excellent in control performance.
A golf ball having a cover including a thermoplastic polyurethane elastomer as a principal component is liable to be spun backwards. When this golf ball is hit by a driver, lift force at an initial stage of the trajectory tends to be too large. Horizontal intensity of the lift force at the initial stage of the trajectory is in a reverse direction to the direction of flight. Excessive backspin impairs the flight distance of the golf ball.
Accordingly, travel performances and control performances are conflicting performances. A golf ball that is satisfactory enough to golf players has not yet obtained. The present invention was made taking into account of such circumstances, and an object of the invention is to provide golf balls which are excellent in both terms of a flight performance and a control performance.